I'd second Bistro de Paris as well as Flying Fish---it depends on the atmosphere you want. From what I understand, FF can be a little loud at peak dinner time. We went for a late dinner (maybe 8:30-9:00) last trip and actually found it was GREAT. We were one of maybe 5 tables with people sitting at it and the food was awesome, I'd definitely recommend their signature red snapper wrapped in potato
As far as Bistro de Paris, we haven't been (I'm lucky to get DH into Chefs de France---is opposed to most things French, don't ask

). That being said, my Aunt and Uncle go to WDW every year and ate there. My Uncle is VERY picky and they both agreed it was the best meal they had ever eaten. That may have been a bit of an exaggeration, but it had to be good for my Uncle to give it the seal of approval.
One additional suggestion would be to "Eat Around the World" instead of going to one restaurant...or doing a different course at a few places. This year, DH and I are going to designate a good 7-8 hours just lingering around the WS. We are going to stop and pick up something to eat, as well as some adult beverages in each country...stopping to explore and take pictures at our leisure. You could try appetizers and main course somewhere like Le Cellier and then stop by the pastry shop in France to pick up dessert and some wine to go, and eat it while you are waiting for Illuminations to begin.
Have a GREAT TIME no matter where you choose, I'm sure you'll appreciate having a nice evening alone.